1. Field of the Invention
The present invention is directed to a neutral electrode for a high-frequency surgery device, and in particular to such an electrode having three sub-electrodes having respective surfaces separated from each other.
2. Description of the Prior Art
A neutral electrode for a high-frequency surgery device is known from U.S. Pat. No. 4,807,621 which has three sub-electrodes arranged on a flexible carrier consisting of insulating material. In several embodiments, the sub-electrodes of this known neutral electrode are each triangular, with two of the sub-electrodes being arranged mirror symmetrically with respect to a symmetry axis, and the third sub-electrode being disposed between the mirror symmetric sub-electrodes. The mirror symmetric sub-electrodes are in the shape of right triangles, so that the combination of all three sub-electrodes results in a rectangular or square configuration. The base of the third sub-electrode proceeds substantially coincident with a line containing the respective apexes of the two right triangles. Conductor runs and terminals for connection of the neutral electrode to external circuitry are also provided on the same side of the neutral electrode on which the sub-electrode surfaces are disposed.
A further neutral electrode having three sub-electrodes is disclosed in U.S. Pat. No. 4,770,173. In this known electrode, two of the sub-electrodes are trapezoids, having edges facing each other which proceed obliquely to a predetermined direction. Another sub-electrode, having a rectangular shape, is disposed in the predetermined direction following the two trapezoidal sub-electrodes. Insulating strips are disposed between the sub-electrodes.
Another neutral electrode arrangement is disclosed in German OS 35 44 443, corresponding to U.S. Pat. No. 4,754,757. This neutral electrode has three sub-electrodes, each of which is rectangular in shape, with the sub-electrodes being disposed parallel to each other on a carrier. A circuit is also disclosed therein for monitoring the surface contact of the neutral electrode with the body of a patient during operation of a high-frequency surgery device. In such high-frequency surgery, misapplication or detaching of the neutral electrode may cause an injurious burn to the patient. A neutral electrode having a plurality of sub-electrodes is therefore provided, connected to a monitoring unit, to trigger an alarm signal to the surgeon as the neutral electrode is becoming detached, but preferably before the neutral electrode becomes detached to such an extent to cause injury to the patient. For this purpose, an auxiliary test current is conducted from a current source to one of the sub-electrodes, through the patient, and returned via another sub-electrode to the test current source. This current is monitored, such as by detecting any changes in impedance. When this circuit is closed, it is insured that each sub-electrode is pressed against the patient with substantially its entire surface, so that the full high-frequency operating current can be applied.
In these known neutral electrodes, however, the arrangement of sub-electrodes unintentionally permits a partial detachment of the sub-electrodes from the surface of the patient without the monitoring circuit responding and thus without the alarm being actuated. This sometimes results in the alarm being triggered too late to avoid injury to the patient. This is because the monitoring circuit reacts differently depending upon the direction of detachment of the neutral electrode from the surface of the patient. It is possible in these known neutral electrodes for the electrode to become partially detached from the body of the patient along a symmetry line so that the changes in impedance between respective pairs of sub-electrodes also change symmetrically, so that an overall impedance difference does not occur, as is necessary to trigger the alarm.
Due to the electrical conductance of the patient tissue, which varies from patient to patient, a resistance or capacitance between two sub-electrodes of a neutral electrode does not, but itself, provide a reliable indicator as to whether contact between the patient and the neutral electrode is sufficient to avoid burning the patient. If, for example, the tissue of one patient has a third of the tissue resistance of another patient, approximately a third of the electrode surface may suffice to meet a preset limit value for the test current given this low tissue resistance. For this reason, the aforementioned neutral electrodes having a plurality of sub-electrodes were devised so that the respective currents between various pairs of sub-electrodes can be measured to determine if detachment has occurred to such an extent to place the patient in danger of being injured. For this purpose, an auxiliary test current can be used, or the high-frequency surgery current. It is assumed that if the currents between the respective pairs of sub-electrodes remain symmetrical, detachment has not occurred. As noted above, however, the known neutral electrodes can become detached from the surface of a patient along a line of symmetry, so that symmetrical test currents are still measured, even though detachment has occurred.